1. Field of the Invention
This invention relates to light-emitting diodes and more particularly to the enhancement of optical polarization of nitride light-emitting diodes by an off-axis cut of the wafer.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., Ref. [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Light-emitting diodes (LEDs) have been used in the last thirty years as indicator lamps, local illuminators, and optical transmitters, among their many applications. In the last ten years, high-brightness aluminum-indium-gallium nitride [(Al,In,Ga)N] based blue and green LEDs have been developed and have started to emerge in general lighting and full-color display applications.
In terms of LED fabrication, because of the incoherent and un-polarized light emission from conventional LEDs, it is not essential to define a particular die orientation of an LED package when the die is attached to the package. In common LED fabrication, die orientation is only significant when an LED wafer is diced, which is why LED photolithographic patterning onto a wafer is carried out by aligning the patterns along crystallographic directions. This alignment process makes die separation reliable and results in higher production yield.
In the case of AlInGaN LEDs prepared on an insulating substrate (for example, sapphire), where two electrical contacts are made on one side of an LED die, die orientation in relation to the package is significant in terms of position of the positive and negative metal contacts. These alignments for reliable die separation and electrical contacts are common practice for any semiconductor devices, not necessarily only in LED fabrication. However, LED die alignment has never been considered in fabrication in terms of emitted light properties.
Internal electrical polarization is a unique property of the AlInGaN system among the semiconductors used in optoelectronics, and this property originates in the hexagonal crystallographic structure of that material system. FIG. 1 schematically shows a generic hexagonal crystal structure 100 with principal crystallographic axes a1 102, a2 104, a3 106, c-axis 108 and typical crystallographic planes of interest (r-plane 110, a-plane 112, m-plane 114, and c-plane 116), with their labeling conventions.
Electrical polarization is created in the hexagonal structure due to its lack of inversion symmetry along the c-axis. For example, in the case of GaN, FIG. 2 illustrates the arrangement of atoms in the GaN crystal 200, wherein open circles 202 represent gallium (Ga) atoms and solid circles 204 represent nitrogen (N) atoms. Along the c-axis 206 of the GaN shown in FIG. 2, Ga atoms 202 (cation, positively charged) and N atoms 204 (anion, negatively charged) are positioned alternately and, as a whole, electric neutrality is maintained. Also shown in FIG. 2 is the a-axis 208.
However, because of the lack of inversion symmetry, there exists an internal electric field along the c-axis when the atoms are displaced from their ideal positions relative to each other along this c-axis. Since atoms in the AlInGaN system usually do not maintain their ideal positions, this polarization field almost always exists along the c-axis. For this reason, the c-plane is called a polar plane. Polarization fields do not exist along any of the a-axes or m-axes, due to the inversion symmetry along these particular axes. For this reason, a-planes and m-planes are called nonpolar planes. For these nonpolar planes, the polarization vector (which expresses direction and strength of the polarization field) is parallel to the planes, since the net polarization vector is parallel to the c-axis.
AlInGaN materials are conventionally grown in the c-direction (i.e., the direction along the c-axis), therefore on the c-plane. LEDs grown on the c-plane show negligible light polarization. On this c-plane, the polarization field has no in-plane component, and the isotropic mechanical stress within the c-plane in a quantum well (QW) structure of an LED does not change the nature of carrier recombination in the QW.
It has recently become possible to prepare AlInGaN LEDs on a- and m-planes. These LEDs exhibit linearly polarized light emission. The polarization field is in a particular direction (c-direction) in the plane, and the stress in the QW is anisotropic due to different degrees of lattice mismatch between the substrate and QW in the two perpendicular directions in the a- or m-plane. The inventors have confirmed that the emitted light from these nonpolar LEDs is linearly polarized in a direction perpendicular to the c-axis. Linearly polarized light is an electromagnetic wave that has its electric field only in one plane perpendicular to the wave's propagation. Non-polarized light has its electric field evenly distributed in directions in planes perpendicular to the wave's propagation. The principal application for polarized light is backlighting for liquid crystal displays (LCDs), for which LEDs are beneficial due to their compactness and energy efficiency as compared to conventional cold cathode fluorescent tubes.
AlInGaN LEDs prepared on a semipolar plane have also been confirmed to emit polarized light. The projection of the polarization vector, which is parallel to the c-axis, lies in the semipolar plane, similar to the nonpolar plane case.
Polarized light emission has been experimentally confirmed from LEDs prepared in nonpolar and semipolar orientations of (Al,In,Ga)N. While all conventional LEDs emit un-polarized light, polarized light emission is believed to be useful in certain applications such as backlighting for LCDs.
To take the advantage of polarized light emission, high polarization ratios are favorable. Thus, there is a need in the art for improved techniques for obtaining high polarization ratios in LEDs.